The complementary metal-oxide-semiconductor (CMOS) image sensor has been recently in the limelight as the next generation image sensor to overcome drawbacks of charged coupled device (CCD) image sensors.
The CMOS image sensor employs a switching method incorporating MOS transistors in unit pixels formed using CMOS technology. A control circuit and a signal processing circuit are used as peripheral circuits and the output of each unit pixel is sequentially detected using the MOS transistors.
That is, the CMOS image sensor reproduces an image by incorporating a photodiode and a MOS transistor in a unit pixel and sequentially detecting an electrical signal of each unit pixel using the switching method.
The CMOS image sensor uses CMOS fabrication technology, and thus has advantages such as low power consumption, a simple fabrication process due to the small number of photolithography processes, etc.
In addition, since the CMOS image sensor can integrate a control circuit, a signal processing circuit, an analog/digital converter circuit, etc. onto a CMOS image sensor chip, it is easy to miniaturize products employing the CMOS image sensor. Accordingly, the CMOS image sensor has been widely used for various products such as a digital still camera and a digital video camera.
FIGS. 1A to 1C are sectional views illustrating a method of fabricating a CMOS image sensor according to a related art.
Referring to FIG. 1A, an insulating layer 101 (for example, an oxide layer) such as for a gate insulating layer or an interlayer insulating layer is formed on a semiconductor substrate 100, and a metal pad 102 for each signal line is formed on the insulating layer 101.
Next, a first overcoat layer 103 is formed on an entire surface of the semiconductor substrate 100 including the metal pad 102, and then a surface of the first overcoat layer 103 is planarized using a chemical mechanical planarization (CMP) process.
Then, a photoresist pattern (not shown) that exposes a portion of the first overcoat layer 103 corresponding to the metal pad 102 is formed on the first overcoat layer 103, and then the first overcoat layer 103 is selectively etched using the photoresist pattern as a mask to expose a surface of the metal pad 102.
Next, a protective layer 104 is formed on an entire surface of the semiconductor substrate including the metal pad 102. The protective layer 104 is formed of an oxide layer or a nitride layer. Also, the protective layer 104 is formed to a thickness of approximately 300-800 Å. The protective layer 104 prevents damage of the metal pad 102 caused by exposing the metal pad 102 to a developing solution during photolithography processes to be performed later.
Then, a color filter resist layer (not shown) is formed on a portion of the protective layer 104 corresponding to a photodiode, and then exposure and developing processes are repeatedly performed to sequentially form red, green, and blue color filters. Such color filters constitute a color filter array 105.
Next, a second overcoat layer 106 is formed on an entire surface of the semiconductor substrate 100 including the color filter array 105, and then is selectively removed except for the portion of the second overcoat layer 106 formed on the photodiode region.
Referring to FIG. 1B, a portion of the protective layer 104 is etched using a reactive ion etch (RIE) method. That is, the protective layer 104 is selectively removed such that the portion of the protective layer 104 located under the color filter array 105 remains.
Here, the second overcoat layer 106 is partially etched due to damage incurred during the etching of the protective layer 104, and thereby creating height differences on the damaged second overcoat layer 106a that is partially etched.
Referring to FIG. 1C, microlenses 107 are formed on the damaged second overcoat layer 106a having the height differences.
However, the height differences of the damaged second overcoat layer 106a change a focal distance of the microlenses 107 and generate a partial difference in thicknesses of the microlenses 107. In addition, this affects topology of the microlenses 107 and the gaps between the microlenses 107, and eventually degrades the electrical characteristic of the CMOS image sensor.
Also, since a surface of the metal pad 102 is exposed after removing the portion of the protective layer 104, the metal pad 102 is exposed to a developing solution during a photolithography process for forming the microlenses 107. The developing solution chemically attacks the metal pad 102, thereby causing a failure or generating a metal piece from the chemically attacked metal pad 102 in the probe test.
The metal piece generated from the metal pad 102 in the probe test may be transferred to the microlenses 107 during the sawing process. The metal piece may then shield or reflect a light incident to the photodiode, which leads to noise generation and defects.